1. Field of the Invention
This invention relates to an optical system for recording and reproducing an optical information medium, and more particularly to an optical system for recording and reproducing an optical information medium, which uses a light source of which oscillating wavelength abruptly changes.
2. Description of the Prior Art
In the recent most general optical system used for an apparatus for recording and reproducing of an optical disk or the like into an optical information recording medium, light issued from a light source 1 is converged on an information recording surface 4 by a collimator or a coupling lens 2 and an objective lens 3 as shown in FIG. 2. The collimator or the coupling lens 2 reduces the divergent light from the light source to a degree of divergence of a parallel light or a level in the proximity thereof, and it is used in the degree of EQU .vertline.1/mc.vertline.&lt;1/4
where mc represents the image-formation magnification. A convergent lens used in the range as described is herein referred to as a collimater. In this optical system, in the case of a surface deviation of an optical disk or the like, focussing is carried out by moving the objective lens 3 in a direction of an optical axis.
The typical objective lens in a reproducing optical system of a compact disk comprises an aspherical lens whose focal length is 4.5 mm, NAO. 45, and made of an acrylic resin, the aspherical lens being put to practical use. On the other hand, a typical collimator lens has a focal length, 17 mm NAO. 14, and those consisting of one group and two lenses are often used. However, both the objective lens and collimator lens are short in correction of chromatic aberration.
Recently, studies have been made to utilize an optical information medium such as an optical disk as a memory medium of memories for large-type electronic computers and personal computers. In the optical system used for that purpose, a typical objective lens has a focal length, 4.5 mm, NAO. 5. Also, the collimator lens used has a focal length 9 mm, NAO. 3, in the case where the light source is a semiconductor laser. This is because of the necessity of effectively taking-in the quantity of light of the semi-conductor laser for carrying out recording.
Generally, the semi-conductor induces a mode hop due to the external environment such as temperatures, and the oscillating wavelength abruptly changes (approximately in the range of .+-.10 nm). Therefore, the focussing servo cannot follow, resulting in a recording error and a reproducing error.
In the compact disk reproducing device, error correcting symbols are contained in the recorded signal so that even if the mode hop should occur, the reproduced sound would not be affected. However, in the case of the memory device, such a memory error and a reproduced error increase the bit error rate (BER) to deteriorate the reliability. Therefor, in the memory device, correction of chromatic aberration need be made so that even if the oscillating wavelength abruptly changes, variation of the focal position is fallen in the range that the servo may follow.
In the case of the objective lens having the focal length of 4.5 mm, NAO. 5, it increases its weight by approximately 1 g. On the other hand, in the memory device of the computer, short access time is important but a heavy objective lens impairs the reduction of the access time.
Japanese Patent Application Laid-open No. 3110/1986 has disclosed an objective lens in an attempt of overcoming such a problem as noted above. However, this lens has problems: (1) centering is difficult because it is a junction lens including an aspherical surface; (2) operating distance is short because the total on-the-axis thickness thereof is larger than that of a single lens due to the one group and two-lens structure; (3) it is difficult to freely select the divergence because material of lenses is restricted to secure surface precision as a press glass lens; and (4) it is expensive because aspherical lenses need be joined together in order to secure an image level.